1. Field of the Invention
The present invention relates to a heat-sensitive recording material having a web-shaped substrate and having a heat-sensitive recording layer disposed to the front side on the web-shaped substrate, the heat-sensitive recording layer comprising at least one dye precursor and at least one (color) developer that reacts with at least one dye precursor. To be reactive with a dye precursor means, in the sense of the present invention in all of its embodiments as herein proposed, that this at least one (color) developer, on sufficient supply of external heat, reacts with this at least one dye precursor to form a visually perceptible printed image. The surface of the heat-sensitive recording material is formed dehesively with respect to layers of adhesive that can be applied on the reverse side of the web-shaped substrate.
2. Description of the Prior Art
Heat-sensitive recording materials giving a color-forming reaction to the supply of external heat have been known for many years and have enjoyed a fundamentally undiminished popularity, a fact attributable among other things to the great advantages attaching to their use for the tradesman issuing tickets and/or sales receipts and/or entry cards. Because the color-forming components—that is, dye precursors and (color) developers, also called color acceptors, which react with said precursors on supply of heat in the case of a heat-sensitive recording process of this kind, reside within the recording material itself, the thermal printers, which are therefore free of toner and ink cartridges, and which no longer require regular servicing, can be installed in large numbers.
Extremely popular in this context are those heat-sensitive recording materials which to the front side have a recording layer, already discussed above in terms of its functional constituents, and to the reverse side have a layer of adhesive that enables the user to employ heat-sensitive recording materials as self-adhesive tickets. This innovative technology has become very comprehensively established to a particular degree in retail—for example, for the pricing of self-weighed products—and also in public transportation—for example, as luggage labels.
Up until the time of their use, the reverse-side layers of adhesive can be covered over by a separate release paper; significantly more popular and also more practical to operate, however, are recording materials which to the front side have surfaces formed dehesively with respect to the reverse-side layers of adhesive. In this case, up until their use, the reverse-side layers of adhesive are covered over by the front-sidedly dehesively formed surfaces of the recording materials themselves, wound up as a roll, where front side and reverse side meet in each case.
Thus DE 44 25 737 A1 explains directly in its first paragraph that in order to produce so-called release papers having dehesive properties with respect to layers of adhesive, it is possible for paper webs to be provided with a silicone layer. In order to prevent penetration of the silicone applied, which is unwanted in that situation, especially penetration of aqueous and above all of solvent-free silicone resins into the paper, a paper web is proposed, for later furnishing with silicone resins, that has been provided with a coating of waterglass. The proposal of a waterglass coating that is found in that specification is, on the one hand, unsuitable for heat-sensitive recording materials; on the other hand, after extensive investigations, the present inventors recognized that even the maximally effective avoidance of silicone penetration into a recording layer situated beneath the silicone furnish is not conducive to the formation of self-adhesive, heat-sensitive tickets.
Again with the objective of preventing fairly unwanted penetration of applied silicone into a base paper, EP 2 239 368 A1 proposes the surface coating of the base paper with subsequent calendering to a surface roughness <100 nm. This specification as well does not relate to heat-sensitive recording materials, and this specification as well, with the objective of extremely effective avoidance of silicone penetration, is oriented in a target direction contrary to the concepts of the invention proposed here.
Subject-matter of EP 0 780 241 B1 is a heat-sensitive recording material whose surface is particularly mechanically stable with respect to various printing processes and also has advantages in terms of water resistance, light stability, and graying. A heat-sensitive recording material proposed therein has a protective layer which features a UV-curing resin and a copolymer resin comprising a silicone component as copolymerizing component. This specification does not address heat-sensitive recording materials for self-adhesive tickets, even if it was found to be suitable for that purpose.
EP 1 637 339 B1 proposes a heat-sensitive recording material comprising a substrate, heat-sensitive recording layer, and protective layer, wherein the protective layer comprises binder resin, filler, crosslinking agent and a release agent. This release agent is introduced as a spherical and particulate silicone compound of specific formula. A disadvantage of this known proposal is the complicated and expensive production, with substrate, recording layer, and final protective layer, wherein the protective layer, because of its filler and binder constituents, surpasses itself by also reducing the dynamic and static sensitivity of the known heat-sensitive recording material with respect to external heat supplied in order to form a visually perceptible printed image.
Documents likewise relevant to the present application are WO 2013/069581 A1 and DE 198 06 433 A1.